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The question does not apply to the Prius because of its planetary power-split device. The engine speed can be anything while the wheel speed is anything. You cannot calculate the engine speed from the road speed. It's not a CVT. It's not a transmission. In fact, the engine will generally be running in its design range, while sending surplus power to the battery via one of the generators. Indeed, the engine speed can be essentially decoupled from the wheel speed if the computer decides to route all the engine's power to one of the MGs, acting as a generator, and then to the other MG, acting as a motor, and thence to the wheels, meanwhile drawing extra power (if needed) from the battery, or sending surplus power (if available) to the battery.

You gotta think in wholly different terms when you think about the Prius. Cool car.

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The Prius transmission almost completely decouples the engine RPM from the wheel RPM, but not quite. The limiting factor is the 10,000 RPM limit on MG1. At highway speeds, this prevents the engine RPM from dropping below idle to zero. So the engine RPM range at highway speeds is 1,000 to 5,000 (limited by the engine, not the transmission). This is for the '04 Prius. For prior years, the maximum engine speed was 4,500 RPM (again, not a transmission limit) and the MG1 limit was 8,000 RPM.

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So, following from that, what is the reaction of the ICE when you suddenly remove your foot from the gas pedal at speed?

Does the ICE simply overrun at zero throttle without changing speed, drop to an idle speed, or fully stop in the case where the vehicle speed is low enough to allow MG1 to compensate by spinning at 10,000 rpm or lower?

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When you suddenly remove your foot from the accelerator pedal (it seems improper to call it a gas pedal), the computer reads a lower number from the hall effect sensors (in the 2004) or the rheostats (in the 2001-03). It quickly adjusts the torque on the larger motor to simulate engine braking (the car has quick response to driver input). It then slowly backs off on the throttle to the engine and reduces its speed over the course of 2 or 3 seconds to avoid the gobs of pollution that gasoline engines create when their load suddenly changes. The engine speed will eventually reach 1000 RPM or zero RPM depending on the speed of the car. In both cases, it will receive no fuel at that point.

So the engine throttle is "fly-by-wire" because the computers continuously select power from both the engine and the battery to meet the current request from the driver.

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Thanks for the reply. One last question: I read somewhere the ICE can be motored in a mode where the valves stay open (for starting) - is that true, and is that the case when the ICE is being motored at 1000 rpm in the above "zero pedal demand" situation?

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One last question: I read somewhere the ICE can be motored in a mode where the valves stay open (for starting) - is that true, and is that the case when the ICE is being motored at 1000 rpm in the above "zero pedal demand" situation?

The intake valves have a fixed dwell (about 270 deg of the 720 deg four-stroke cycle), but the start of open can be adjusted within a 43 deg range (2001-03 Prius, I've heard, but not verified that it's a smaller range in the 2004). The exhaust valve timing is fixed. So, no, the valves cannot be held open throughout the 720 degree cycle thus there is no way to eliminate pumping losses. To somewhat reduce pumping losses in these situations, the beginning of the intake valve opening is set to overlap the end of the exhaust valve opening, thereby increasing exhaust gas recirculation.

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